Is it possible to have sims have their own personal money inventory thing? Sorry, not sure where to ask this. For example, I have my sim Onwei living in an apartment seperate from her family. She's still in the family, but I was wondering if she could have her own finances? Thanks in advance.
Also, I tried adding someone to the portrait panel and then making them selectable and they still aren't selectable. If I do this, will they show their own household funds or not?

The only way I know to keep money separate for each indiviual sim is by letting them have their own bank account. This will allow your sims to deposit, withdraw and transfer money from their account. You can find the mod for this here. I use it all the time and it works great

Awesome! Thank you! 
one more question: 
So I have a young adult daughter Sim in my family. I want her to move out on her own basically but stay part of the family at the same time. I've purchased a second home (apartment building) and have her living there, but I'd like her to have her own family funds. What's the best way to do that? 
A. Have her move out, then try to use master controller and portrait panel to make her selectable? (Have tried testing this on other Sims and doesn't seem to work. 
B. Have her move out, then add her as a roommate and make her selectable? I don't mind her paying her own rent because then she's paying rent on her little apartment, but would she still be able to access the apartment if she's not part of the active family?
C. Open a bank account for her
D. Some other option I'm unaware of

Sims can only be selectable if they are part of the household being played. PortraitPanel (the mod) can be used to switch active households if that's what you wanted, as can Selector among others. But these are just shortcuts for switching households with the same result as using MasterController to switch.

There is no such thing as a selectable "roommate." Roommates in the formal game sense are NPCs and the homeless NPC "household" cannot be playable.

I thought I had seen the option to ask sims to be my roommate???

Yes, you can ask a sim to be a roommate and they will move into your home, but they remain unelectable and do there own thing... as far as I know. Your family can still interact with them, call to meal etc but the roommate is unelectable. Like having roommates at a dorm in university.

for the banking, wherever your sim moves to their bank account stays with them.

When you make a sim a roommate either by in-game social interaction (Ask to Be Roommate) or by way of MC, you are making them into an NPC. A Non-Playable Character who has no household nor finances of their own.

It is not possible in TS3 to play sims from more than one household without switching active households back and forth. Again, the notion of selectability only applies to the actively being played household. It sounds like you may need to rethink how to proceed with this one.

I had thought that while I was in college i had made my roommates selectable, or seen something about it, but I guess I was remembering wrong. That's what I thought the "make selectable" button was for on nraas. 
Things are currently fine with one of the sims living in the additional home. Well, except for the fact that I sort of broke things by fooling around testing selectability on a nonactive sim, and made the game think he owned the place and had to evict him. Prior to that I could access the fridge and the bookshelves normally, and the fridge allowed me to make foods and have access to the home lots food inventory. Now I can't cook, only get snacks. Since I evicted him I have access to the home lots inventory, but can't cook. I tried deleting the fridge and adding a new one... didn't work. The bookshelf used to work now it functions similarly to a library shelf. Extremely frustrating. I wish I had realized what had happened before I saved the game, but it's too late now and the only solution I can think of at this point is to purchase ANOTHER additional home and sell this one and hope THAT resolves things, although one sim is about to go off to college so maybe somehow that will fix things..... ...... ...... and if not, I'll do what I previously mentioned, and just have the two sisters get a place together or something. 
I wish the "purchase additional home" option worked more like you'd think it would, but I'd still prefer to try and use it to having my sim live on the family lot. She hasn't had any issues with her bed, even though she can't "claim" it, she will still use it. I don't intend her to have children, and if she does, she'll just move back home to raise them anyway. All in all for a single young adult sim who never intends to marry it's pretty useful and I'm delighted it works in whatever form it does. I was only exploring possible other solutions to enhance it. Thank you for your responses, they're greatly appreciated. If anyone ever makes a mod that fixes the "purchase additional home" settings to make it function more as one would expect, I'd be happy to test it. 

Whenever my game gets messed up I try a few things.
1. Fix homeworld: From City Hall >Nraas>DE>Options:City Hall>Sim>Fix Homeworld. (I do this everythime I travel there and back)
2. Reset Everything: Not in game at the moment so not sure of that path. I thinks its in MC
3. Reset All Sims: Open the cheat box and type in resetsim *

See if any of those help.

Thanks, I will try that!